The source of abnormal outflow resistance in primary open angle glaucoma (POAG), a leading cause of blindness, has yet to be identified. We have found that acute elevation of intraocular pressure (IOP) in normal bovine and human eyes causes a decrease in the available area for aqueous outflow, Schlemm's canal (SC) to collapse and reversible herniations into collector channel (CC) ostia. In contrast, herniations were commonly observed in human POAG eyes even at 0 mmHg, suggesting that a normally reversible process of hernia formation becomes irreversible in the diseased state. A number of herniated regions in POAG eyes appear denser and with local accumulation of extracellular matrix compared with the herniated regions following acute IOP elevation in normal eyes. We propose that as more irreversible herniations develop in the CC ostia, the outflow facility would progressively decrease and IOP would rise. Critical data supporting this hypothesis comes from clinical studies demonstrating that flow of fluorescein or trypan blue injected into SC of POAG eyes is blocked from entering the episcleral veins in certain regions. A poorer prognosis following a non- penetrating glaucoma surgery in POAG eyes seems to be associated with increased numbers of these blockages. Our goal is to determine whether the irreversible herniations in POAG eyes we have identified account for the blockage of the fluorescein or trypan blue egress from SC into episcleral veins observed clinically in POAG patients and the physiological significance of these findings in the pathogenesis of POAG and in the prognosis of non-penetrating glaucoma surgery. We hypothesize that the decreased outflow facility in POAG eyes is associated with a decrease in effective filtration area in the juxtacanalicular connective tissue and inner wall of SC, and is proportionally associated with an increased number of irreversible herniations into CC ostia. We hypothesize that these herniations are responsible for blocking the fluorescein or trypan blue from entering the episcleral veins from SC in clinical studies. To investigate this hypothesis three specific aims are proposed: 1. Distinguish the hydrodynamic and structural differences between the reversible herniations into CC ostia observed in normal human eyes with acute IOP elevation and irreversible herniations in POAG eyes. 2. Determine whether the blockage of trypan blue or fluorescein entering the episcleral veins from SC observed clinically is caused by irreversible herniations in glaucomatous eyes compared to normal eyes. This will be examined histologically by tracing nanoparticles through these herniated regions after in vivo injection in the naturally occurring and laser-induced glaucomatous monkey eyes. 3. Establish an egress pattern of dye from SC to episcleral veins as it appears in normal eyes, termed "channelography" and investigate its common changes in POAG patients undergoing a non-penetrating glaucoma surgery and its relationship to post- operative IOP reduction. If we can confirm that irreversible herniations into CC ostia are indeed the site blocking the aqueous outflow, a new site of added resistance in POAG will be identified. PUBLIC HEALTH RELEVANCE: Primary Open-angle Glaucoma is a major cause of permanent blindness, found in approximately 2% of individuals over the age of 40, and affects over 70 million people in the world. The goal of our research is to identify the pathologic factors contributing to elevated outflow resistance in primary open angle glaucoma.